Food Biochemistry and Food Processing (2 edition)

BLBS102-c25 BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come

476 Part 4: Milk

products, such as processed cheese, cheese analogues and

enzyme-modified cheese (EMC). Processed cheese is produced

by blending natural cheeses of differing stages of ripening,

to balance structural and flavour properties, and adding wa-

ter and emulsifying salts that convert the insoluble calcium

para-caseinate to more soluble and amphiphilic sodium para-

caseinate, which then produces a stable emulsion. Emulsification

is carried out under high-shear conditions at 70–90◦C, yielding a

safe and stable product. Following this molten stage, the cheese

can be set (on cooling) into a wide range of shapes and forms,

including blocks and slices; in addition, by choosing the formu-

lation and ingredients used, a range of textures from solid blocks

to spreads, which can have a range of added food components

and flavours, can be produced.

In the case of cheese analogues, natural cheese is not used, and

the main source of casein is usually rennet casein; such products

may be used as Mozzarella substitutes for pizza toppings. EMC

is cheese curd that has been incubated with enzymes (e.g., pro-

teinases, peptidases and lipases) to yield a concentrated product

with an intense cheese flavour that can be used as an ingredient

(often in dried form) in food formulations. EMC with flavour

profiles matching different cheese varieties have been produced.

Acid-Coagulated Cheeses

Acid-coagulated cheeses are at one end of the spectrum of fer-

mented dairy products, the production of which is summarised

in Figure 25.2A. Depending on the desired fat content of the final

product, the starting material may be low-fat cream, whole milk,

semi-skimmed milk or skimmed milk. The milk for Cottage-

type cheese is subjected to only mild heat treatment so that the

syneretic properties of the coagulum are not impaired.

Fermented Milks

In addition to acid-coagulated cheese, a wide range of fermented

milk products are produced worldwide today, including:


 products of fermentation by mesophilic or thermophilic

LAB (e.g., yoghurt); and


 products obtained by alcohol-lactic fermentation by LAB

and yeast (e.g., kefir, koumiss).

For yoghurt, as for acid-coagulated cheese, the milk base

(with a fat content depending on the final product) is usually

supplemented with milk solids to enhance the viscosity and

rigidity of the final coagulum, in some cases to simulate the

viscosity of yoghurt made from sheep’s milk, which has a higher

total solids content than cows’ milk. The milk solids added may

be WMP or SMP or whey powders, depending on the type of

product required (Gonz ́alez-Mart ́ınez et al. 2002, Remeuf et al.

2003). The most commonly used milk powder today is probably

SMP, typically of medium heat classification. Buttermilk powder

may be used also as a substitute for SMP, with the advantages

of good emulsifying properties and potential biological activity.

Syneresis, which causes free whey in the product, is very

undesirable in fermented milk products. Therefore, the milk or

starting mix is subjected to a severe heat treatment (e.g., 90◦Cfor

10 minutes) to denature the whey proteins, increase their water-

binding capacity and produce a firmer gel network, consist-

ing of casein–whey protein complexes. The heat treatment also

kills vegetative pathogenic bacteria, rendering the product safe

(Robinson 2002a). Homogenisation of the mix also increases

the firmness of the gel, because the fat globules in homogenised

milk are stabilised mainly by a surface layer of adsorbed casein

micelles, through which they can become structural elements of

the final acid-induced gel.

The relative proportion of caseins and whey proteins in yo-

ghurt has a significant influence on the structure of the gel

(Puvanenthiran et al. 2002). Hydrocolloid stabilisers (polysac-

charides or proteins) may be added to the mix to modify the

viscosity (by gelling or thickening) and/or prevent syneresis

(Fizsman et al. 1999, Tamime and Robinson 1999). Pectin is

probably the most commonly used hydrocolloid stabiliser for

yoghurt, but carageenan, guar gum, alginate and various modi-

fied and natural starches are also widely used. Sweetening agents

(e.g., sugars or synthetic sweeteners such as aspartame) may be

added to the mix, and fruit purees or fruit essence may be added

either at the start or end of fermentation.

The starter culture for yoghurt containsStr. thermophilusand

Lb. bulgaricussubsp.bulgaricus. In recent years, many fer-

mented milk products have been introduced with added probi-

otic bacteria such asBifidobacteriaspp. orLactobacillus aci-

dophilus,due to increasing consumer interest in the health ben-

efits of such bacteria, and increasing clinical evidence of their

efficacy (Surono and Hosono 2002, Robinson 2002b, Reid 2008,

Sanchez et al. 2009). The texture and viscosity of yoghurt may

be modified by using an exopolysaccharide (EPS)-producing

starter culture (strains of bothStr. thermophilusandLb. bulgar-

icussubsp.bulgaricushave been identified that secrete capsular

heteropolysaccharides; De Vuyst et al. 2001, Hassan et al. 2003).

During fermentation, the production of lactic acid destabilises

the casein micelles resulting in their coagulation (acid-induced

gelation) (Lucey and Singh 1998). The main point of differ-

ence between the production protocols for yoghurt and acid-

coagulated cheese is that when the pH has reached 4.6 and the

milk has set into a coagulum, this coagulum is cut or broken for

the latter but not for the former. In addition, the heat treatment

applied to milk for the manufacture of acid-coagulated cheese is

less severe than for yoghurt, to facilitate syneresis in the former,

while avoiding it in the latter.

Depending on the desired characteristics of the finished prod-

uct, the milk may be fermented in the final package (to produce

a firm set yoghurt) or in bulk tanks, followed by filling (to pro-

duce a more liquid stirred yoghurt) (Tamime and Robinson 1999,

Robinson 2002a). The rate of acidification for set or stirred yo-

ghurt differs due to differences in the level of inoculation and

incubation temperature.

It is important to cool the yoghurt rapidly when pH 4.6 is

reached, to preserve the gel structure and prevent further starter

activity (Robinson 2002a). In some cases, the viscosity of the

yoghurt may be reduced by post-fermentation homogenisation

to produce drinking yoghurt.

Yoghurt has a relatively short shelf life; common changes

during storage involve syneresis, with whey expulsion, and